In order to make use of metals, they first must be extracted from raw materials, such as metal oxides, or from waste streams. One particular problem is how to extract metals from metal oxides, while minimizing environmental contamination. Metals currently are liberated from metal oxides by first crushing the oxide and then treating the crushed material with an acid that dissolves the metals. Acid dissolution is followed by selective precipitation, electrowinning, or solvent extraction. Acid dissolution is unfortunately very nonspecific, and often produces many by-products, including aqueous and organic wastes, that create serious environmental problems in their own right.
It also is known to extract metal ions from materials using fluid extraction, such as supercritical fluid extraction. A supercritical fluid is typically one that is gaseous at ambient conditions, but which is maintained at a temperature and pressure above its critical temperature and pressure. Supercritical solvents can be used to extract organic materials, such as caffeine from coffee beans. U.S. Pat. No. 4,911,941 provides an example of supercritical carbon dioxide extraction of caffeine in which green coffee beans are moved periodically through an extraction vessel and contacted with continuously flowing supercritical carbon dioxide. U.S. Pat. No. 4,898,673 shows a similar system in which soluble materials are continuously extracted from solids using supercritical carbon dioxide. The soluble solids are circulated in a closed-loop pipeline with the supercritical fluid.
Supercritical extraction of environmental wastes is disclosed in Wai et al.'s U.S. Pat. No. 5,356,538, as well as Wai et al.'s U.S. Patent Application entitled FLUID EXTRACTION, filed Jun. 2, 1995, which is a continuation-in-part of co-pending U.S. patent application Ser. No. 08/253,000, filed Jun. 2, 1994 (Wai's patent documents). Wai's patent documents, which are incorporated herein by reference, disclose the extraction of metalloid and metal ions from materials by exposing the material to a fluid solvent, particularly supercritical carbon dioxide, containing a chelating agent.
Directly extracting metals from metal oxides apparently is not known. As stated above, known methods involve dissolving crushed metal oxides in a strong acid, such as concentrated nitric acid, before extraction to separate the metal from the metal oxide. This is done to disrupt the metal oxide matrices which possess high structural stability. This characteristic of metal oxides has discouraged efforts at direct extraction of metals from metal oxides in the past.
A need exists, therefore, for an environmentally safe method for directly extracting metals from metal oxides while avoiding intermediate processing steps.